Embodiments of the present invention relate to a Fault Current Limiter (FCL). In particular, they relate to an FCL for an electricity distribution network.
Electricity distribution networks are used to transmit electric current from a source to a load. If an electrical fault occurs in such a network, large currents are generated that can cause damage to the load. Fault current limiters vary their impedance when there is a fault and reduce the current.
Superconducting materials have a low impedance superconductive state and a high impedance normal state. They can be used as a variable impedance element, in a fault current limiter. Under usual operating conditions, the superconducting element has approximately zero impedance and therefore no voltage is dropped across the FCL. When a fault occurs and a large current starts to flow, the superconducting element switches from its superconductive state to a normal state, introducing impedance into the network. A significant voltage is consequently dropped across the FCL.
Low temperature superconductors (LTS) have a transition temperature less than 23.2K. The transition temperature (Tc) is the temperature below which it is possible for the material to superconduct. The material superconducts when the applied magnetic field, the current density and the temperature are all below threshold values dependent upon the material type. Liquid helium must be used to cool a LTS below its transition temperature (Tc) but it is very costly and is an inefficient coolant. This makes an LTS expensive to use in a FCL.
Ceramic-based high temperature superconducting (HTS) materials were developed in the 1980's. HTS materials can superconduct at temperatures attainable with the use of liquid nitrogen (77 K). Liquid nitrogen is approximately 20 times more effective at cooling than liquid helium and 10 times less expensive. However, HTS materials are difficult to manufacture because of their mechanical weakness and brittle nature.
DE 199 04 822 C describes an arrangement in which liquid nitrogen is used to cool the current leads of a load such as a current limiter. The nitrogen is cooled in a Joule-Thompson refrigerating machine to a temperature of 77K and is then conveyed along the current leads in a direction counter to the incoming heat.
Therefore, it would be desirable to provide a superconductive Fault Current Limiter that can use liquid nitrogen as a coolant but has greater mechanical strength than the HTS materials.